Oxide Materials for Thermoelectric Conversion

Thermoelectric technology has emerged as a prominent area of research in the past few decades for harnessing waste heat and improving the efficiency of next-generation renewable energy technologies. There has been rapid progress in the development of high-performance thermoelectric materials, as measured by the dimensionless figure of merit (ZT = S2 · σ · κ−1). Several heavy-metal-based thermoelectric materials with commercial-level performance (ZT = 1) have so far been proposed. However, the extensive application of these materials still faces challenges due to their low thermal/chemical stability, high toxicity, and limited abundance in the Earth’s crust. In contrast, oxide-based thermoelectric materials, such as ZnO, SrTiO3, layered cobalt oxides, etc., have attracted growing interest as they can overcome the limitations of their heavy-metal-based counterparts. In this review, we summarize the recent research progress and introduce improvement strategies in oxide-based thermoelectric materials. This will provide an overview of their development history and design schemes, ultimately aiding in enhancing the overall performance of oxide-based thermoelectric materials.


Introduction
Following the progression of human civilization and industrialization, vast amounts of fossil fuels have been consumed for energy purposes [1]. However, due to the low efficiency of utilizing fossil fuels, only around 30% of the primary energy can be effectively harnessed, while more than 60% is wasted as heat [2]. Therefore, the exploration of new and efficient technologies for recycling waste heat is crucial to align with the sustainable world envisioned by the United Nations in 2015 [3].
Thermoelectrics has emerged as a promising technique for the reutilization of waste heat [4,5]. By leveraging the Seebeck effect, thermoelectric materials can directly convert a temperature gradient into electricity (Figure 1a) [6]. The performance of a thermoelectric material is commonly quantified by a dimensionless figure of merit known as ZT, which is defined as: κ where S, σ, κ, and T denote the Seebeck coefficient (or thermopower), electrical conductivity, thermal conductivity, and absolute temperature, respectively. The overall conversion efficiency (η) of a thermoelectric generator can be determined by ZT using the following relationship [1]: where T h , T c , and ∆T are the temperature at the hot and cold sides, and the temperature gradient along the thermoelectric generator, respectively. In accordance with the second law Thus far, a range of thermoelectric materials exhibiting high ZT values has been discovered, including filled skutterudites alloy [8], semi-Heusler intermetallic compounds [9], Bi2Te3 [10], Mg3Sb2 [11], GeTe [12], AgPbmSbTe2+m [13], Cu2Se [14], Cu2S [15], SnSe [16][17][18][19][20], etc, primarily belonging to the chalcogenide family. However, despite advancements, the conversion efficiency of thermoelectric materials still falls short of meeting practical application requirements when compared to traditional energy conversion techniques [7]. Additionally, these materials face limitations in high-temperature usage (i.e., > 700 °C) due to their low thermal and chemical stabilities. Furthermore, their low natural element abundance and high toxicity introduce uncertainties and challenges in their practical implementation.
To address these challenges, considerable attention has been directed toward oxidebased thermoelectrics as a potential solution. Oxide-based thermoelectric materials offer higher thermal and chemical robustness, making them more suitable for high-temperature applications. Moreover, as depicted in Figure 1b, the advantage of operating at high temperatures can offset some of the limitations associated with lower ZT values. Using a temperature gradient applied along the thermoelectric P-N junction, charge carriers inside the materials are driven from the hot side to the cold side, resulting in a current flow (I) through the circuit. (b) Power generation efficiency (η), as a function of the temperature of heat source (T h ), in the thermoelectric generators with different average ZT values (ZT avg ). The efficiency values for several heat engines currently being used are plotted for comparison. Reproduced with permission [7].
To address these challenges, considerable attention has been directed toward oxidebased thermoelectrics as a potential solution. Oxide-based thermoelectric materials offer higher thermal and chemical robustness, making them more suitable for high-temperature applications. Moreover, as depicted in Figure 1b, the advantage of operating at high temperatures can offset some of the limitations associated with lower ZT values.

ZnO
ZnO is one of the most investigated oxides for photovoltaic, sensing, piezoelectric, and thermoelectric applications, with n-type semiconducting behaviors [34,[47][48][49][50]. As shown in Figure 2a, ZnO crystals possess a wurtzite structure, where the Zn atom is surrounded by four oxygens, resulting in a hexagonal closely packed sublattice. In the solid state, ZnO acts as an insulator with a wide bandgap of about 3.2-3.5 eV. The conduction band minimum (CBM) is mainly derived from the 4s orbital in the Zn 2+ ion, which shows a high carrier mobility. Pristine ZnO displays an increasing electrical conductivity with temperature due to the thermally excited electrons. Under ideal stoichiometric conditions, ZnO materials demonstrate a thermoelectric power factor ranging from 0.8 to 1 mW m −1 K −2 . However, due to the low electrical conductivity and high thermal conductivity (49 W m −1 K −1 at 300 K and 10 W m −1 K −1 at 1000 K [51]), the ZT value and overall efficiency of pristine ZnO are still limited in practical applications.

ZnO
ZnO is one of the most investigated oxides for photovoltaic, sensing and thermoelectric applications, with n-type semiconducting behaviors shown in Figure 2a, ZnO crystals possess a wurtzite structure, where the Z rounded by four oxygens, resulting in a hexagonal closely packed sublatti state, ZnO acts as an insulator with a wide bandgap of about 3.2-3.5 eV. T band minimum (CBM) is mainly derived from the 4s orbital in the Zn 2+ ion a high carrier mobility. Pristine ZnO displays an increasing electrical con temperature due to the thermally excited electrons. Under ideal stoichiome ZnO materials demonstrate a thermoelectric power factor ranging from 0 K −2 . However, due to the low electrical conductivity and high thermal cond m −1 K −1 at 300 K and 10 W m −1 K −1 at 1000 K [51]), the ZT value and overa pristine ZnO are still limited in practical applications. In order to enhance the electrical characteristics of zinc oxide (ZnO), th of donor dopants such as Al 3+ and Ga 3+ has been widely employed. A sm dopants can tune the conduction behavior of ZnO from semiconducting 1996, Ohtaki and Tswbota reported on the high-temperature thermoelectri Al-doped ZnO, exhibiting a thermoelectric figure of merit (ZT) of appro 1273 K, along with a thermoelectric power factor ranging from 1 to 1.5 mW is comparable to traditional chalcogenide-based counterparts [34,50]. In 2 prepared ZnO nanocomposites incorporating Al, further enhancing the Z at 1000 K [53]. In addition to Al, a diverse range of dopants, including Ga, Ti, Mn, and Sb, have been utilized to improve either the overall or specifi mance characteristics [54][55][56]. Furthermore, apart from their influence on e erties, elemental doping has proven effective in reducing the thermal cond ZnO system. As shown in Figure 3, Ohtaki et al. found that using a third e doped with Al, could dramatically reduce the thermal conductivity, w In order to enhance the electrical characteristics of zinc oxide (ZnO), the introduction of donor dopants such as Al 3+ and Ga 3+ has been widely employed. A small number of dopants can tune the conduction behavior of ZnO from semiconducting to metallic. In 1996, Ohtaki and Tswbota reported on the high-temperature thermoelectric properties of Al-doped ZnO, exhibiting a thermoelectric figure of merit (ZT) of approximately 0.3 at 1273 K, along with a thermoelectric power factor ranging from 1 to 1.5 mW m −1 K −2 , which is comparable to traditional chalcogenide-based counterparts [34,50]. In 2011, Jood et al. prepared ZnO nanocomposites incorporating Al, further enhancing the ZT value to 0.44 at 1000 K [53]. In addition to Al, a diverse range of dopants, including Ga, Ni, Co, Fe, In, Ti, Mn, and Sb, have been utilized to improve either the overall or specific ZnO performance characteristics [54][55][56]. Furthermore, apart from their influence on electrical properties, elemental doping has proven effective in reducing the thermal conductivity in the ZnO system. As shown in Figure 3, Ohtaki et al. found that using a third element, Ga, co-doped with Al, could dramatically reduce the thermal conductivity, while having a relatively small effect on electrical conductivity [57]. With an obvious reduction in thermal conductivity and enhancement in power factor, the ZT value of the double-doped oxide in the composition of Zn 0.96 Al 0.02 Ga 0.02 O is 0.47 at 1000 K and 0.65 at 1247 K.
Molecules 2023, 28, 5894 4 of 18 relatively small effect on electrical conductivity [57]. With an obvious reduction in thermal conductivity and enhancement in power factor, the ZT value of the double-doped oxide in the composition of Zn0.96Al0.02Ga0.02O is 0.47 at 1000 K and 0.65 at 1247 K. In 2021, utilizing the thermal conductivity reduction effect of the coating grain structure, Somnath Acharya et al. prepared ZnS-coated Al-doped compact ZnO nanostructures using the low-temperature co-precipitation method and spark plasma sintering (SPS), in which the Al doping increased the carrier concentration as well as optimizing the electron transport characteristics ( Figure 4). The decrease in thermal conductivity can be attributed to the enhanced coherent phonon-scattering in Zn1−xAlxO. The power factor of the resulting 2% ZnS-coated Zn0.98Al0.02O nanostructures exhibited a noteworthy enhancement of 161% compared to pure ZnO, reaching a value of 0.75 mW m⁻ 1 K⁻ 2 . Furthermore, a peak figure of merit (ZT) value of 0.2 was attained, which represents a remarkable 272% increase compared to pure ZnO [58].  In 2021, utilizing the thermal conductivity reduction effect of the coating grain structure, Somnath Acharya et al. prepared ZnS-coated Al-doped compact ZnO nanostructures using the low-temperature co-precipitation method and spark plasma sintering (SPS), in which the Al doping increased the carrier concentration as well as optimizing the electron transport characteristics ( Figure 4). The decrease in thermal conductivity can be attributed to the enhanced coherent phonon-scattering in Zn 1−x Al x O. The power factor of the resulting 2% ZnS-coated Zn 0.98 Al 0.02 O nanostructures exhibited a noteworthy enhancement of 161% compared to pure ZnO, reaching a value of 0.75 mW m −1 K −2 . Furthermore, a peak figure of merit (ZT) value of 0.2 was attained, which represents a remarkable 272% increase compared to pure ZnO [58]. relatively small effect on electrical conductivity [57]. With an obvious reduction in thermal conductivity and enhancement in power factor, the ZT value of the double-doped oxide in the composition of Zn0.96Al0.02Ga0.02O is 0.47 at 1000 K and 0.65 at 1247 K. In 2021, utilizing the thermal conductivity reduction effect of the coating grain structure, Somnath Acharya et al. prepared ZnS-coated Al-doped compact ZnO nanostructures using the low-temperature co-precipitation method and spark plasma sintering (SPS), in which the Al doping increased the carrier concentration as well as optimizing the electron transport characteristics ( Figure 4). The decrease in thermal conductivity can be attributed to the enhanced coherent phonon-scattering in Zn1−xAlxO. The power factor of the resulting 2% ZnS-coated Zn0.98Al0.02O nanostructures exhibited a noteworthy enhancement of 161% compared to pure ZnO, reaching a value of 0.75 mW m⁻ 1 K⁻ 2 . Furthermore, a peak figure of merit (ZT) value of 0.2 was attained, which represents a remarkable 272% increase compared to pure ZnO [58].  More recently, as shown in Figure 5, Soumya Biswas et al. prepared ZnO nanocomposites using a simple solution-based method with both Al doping and reduced graphene oxide (RGO) encapsulation techniques. The influence of Al doping on the nanocomposites resulted in an increased electrical conductivity and decreased thermal conductivity, albeit accompanied by low Seebeck coefficient values. Furthermore, the incorporation of RGO inclusions into the nanocomposites yielded dual benefits. Firstly, it enhanced the overall electrical conductivity, contributing to improved charge transport properties. Secondly, through the phenomenon of energy filtration, the presence of RGO enhanced the Seebeck coefficient, promoting more favorable thermoelectric properties. Consequently, the synthe-sized material exhibited a notable figure of merit (ZT) value of approximately 0.52 at an elevated temperature of 1100 K [59]. accompanied by low Seebeck coefficient values. Furthermore, the incorporation inclusions into the nanocomposites yielded dual benefits. Firstly, it enhanced th electrical conductivity, contributing to improved charge transport properties. S through the phenomenon of energy filtration, the presence of RGO enhanced the coefficient, promoting more favorable thermoelectric properties. Consequently, thesized material exhibited a notable figure of merit (ZT) value of approximatel an elevated temperature of 1100 K [59]. In addition to the bulk state, recent progress has also been achieved in two sional thin film-based ZnO materials. Zhou et al. prepared Ga-doped ZnO epita films using pulsed laser deposition (PLD) technology. During the preparation growth rate, crystallinity, surface roughness, and even the Fermi level could be by controlling deposition temperature. The improved Ga doping efficiency resul increased Hall mobility and higher carrier concentration, consequently, leading t ble enhancement in electrical conductivity. Finally, GZO films deposited at 673 K the best quality and crystallinity, with a power factor of 256 mW m −1 K −2 [60]. Shimizu et al. proposed that the thermoelectric performance of ZnO could be e by a two-dimensional electron gas in the ion gel-gated field-effect transistor (Fi [61]. The accumulation of an ~1 nm 2D electron gas at the surface of ZnO can sign enhance the Seebeck coefficient (Figure 6b), which brings about an obviously i power factor compared to the bulk counterparts [62][63][64][65]. In addition to the bulk state, recent progress has also been achieved in two-dimensional thin film-based ZnO materials. Zhou et al. prepared Ga-doped ZnO epitaxial thin films using pulsed laser deposition (PLD) technology. During the preparation process, growth rate, crystallinity, surface roughness, and even the Fermi level could be changed by controlling deposition temperature. The improved Ga doping efficiency resulted in an increased Hall mobility and higher carrier concentration, consequently, leading to a notable enhancement in electrical conductivity. Finally, GZO films deposited at 673 K showed the best quality and crystallinity, with a power factor of 256 mW m −1 K −2 [60]. In 2016, Shimizu et al. proposed that the thermoelectric performance of ZnO could be enhanced by a two-dimensional electron gas in the ion gel-gated field-effect transistor (Figure 6a) [61]. The accumulation of an~1 nm 2D electron gas at the surface of ZnO can significantly enhance the Seebeck coefficient (Figure 6b), which brings about an obviously increased power factor compared to the bulk counterparts [62][63][64][65].

SrTiO3
SrTiO3 is considered a promising n-type oxide thermoelectric material with a high Seebeck coefficient due to the large effective mass of the Ti 3D orbital [43]. Meanwhile, its high melting point also makes it a thermoelectric candidate at high temperatures. As shown in Figure 7, SrTiO3 has a simple cubic-perovskite crystal structure (lattice parameter, a = 3.905 Å). SrTiO3 behaves as an insulator, characterized by a very low carrier concentration, poor electron transport, and high thermal conductivity. Therefore, to enhance the thermoelectric performance of SrTiO3, two crucial aspects necessitate attention: optimizing the carrier concentration and reducing thermal conductivity. SrTiO3 possesses a high relative dielectric constant (εr~220 at room temperature), and the impurity orbitals start to overlap at a rather low donor doping level (~0.5 mol%), resulting in a metallic electron transport property in SrTiO3 [43]. In this case, it is easy to introduce dopants (i.e., La 3+ and Nb 5+ ) into the SrTiO3 matrix and modulate the thermoelectric properties.

SrTiO 3
SrTiO 3 is considered a promising n-type oxide thermoelectric material with a high Seebeck coefficient due to the large effective mass of the Ti 3D orbital [43]. Meanwhile, its high melting point also makes it a thermoelectric candidate at high temperatures. As shown in Figure 7, SrTiO 3 has a simple cubic-perovskite crystal structure (lattice parameter, a = 3.905 Å). SrTiO 3 behaves as an insulator, characterized by a very low carrier concentration, poor electron transport, and high thermal conductivity. Therefore, to enhance the thermoelectric performance of SrTiO 3 , two crucial aspects necessitate attention: optimizing the carrier concentration and reducing thermal conductivity. SrTiO 3 possesses a high relative dielectric constant (ε r~2 20 at room temperature), and the impurity orbitals start to overlap at a rather low donor doping level (~0.5 mol%), resulting in a metallic electron transport property in SrTiO 3 [43]. In this case, it is easy to introduce dopants (i.e., La 3+ and Nb 5+ ) into the SrTiO 3 matrix and modulate the thermoelectric properties. In addition to the common donor dopant La 3+ and Nb 5+ , other elements are also utilized to tailor the carrier concentration, lattice constant, defect level, and lattice distortion. In the A-site doping, common elements included La, Y, Na, Ce, Mn, Bi, Nd, Pr, Sm, Gd, Dy, etc. [5,66]. At the B-site, common doping elements included Nb, Ta, Se, V, Cr, Fe, Co, Ni, In, Sb, Mg, etc. Ru, Al, etc. [67]. In 2001, Okuda first found that the power factor of  In addition to the common donor dopant La 3+ and Nb 5+ , other elements are also utilized to tailor the carrier concentration, lattice constant, defect level, and lattice distortion. In the A-site doping, common elements included La, Y, Na, Ce, Mn, Bi, Nd, Pr, Sm, Gd, Dy, etc. [5,66]. At the B-site, common doping elements included Nb, Ta, Se, V, Cr, Fe, Co, Ni, In, Sb, Mg, etc. Ru, Al, etc. [67]. In 2001, Okuda first found that the power factor of Sr 1−x La x TiO 3 was 28-36 mW cm −1 k −2 , which is comparable to traditional alloy-type thermoelectric material Bi 2 Te 3 at room temperature, although its ZT was still <1 due to its excessive thermal conductivity (Figure 8). Despite this limitation, the notable power factor exhibited by Sr 1−x La x TiO 3 proved its potential for application in thermoelectric systems [42]. In addition to the common donor dopant La 3+ and Nb 5+ , other elements are also utilized to tailor the carrier concentration, lattice constant, defect level, and lattice distortion. In the A-site doping, common elements included La, Y, Na, Ce, Mn, Bi, Nd, Pr, Sm, Gd, Dy, etc. [5,66]. At the B-site, common doping elements included Nb, Ta, Se, V, Cr, Fe, Co, Ni, In, Sb, Mg, etc. Ru, Al, etc. [67]. In 2001, Okuda first found that the power factor of Sr1−xLaxTiO3 was 28-36 mW cm −1 k −2 , which is comparable to traditional alloy-type thermoelectric material Bi2Te3 at room temperature, although its ZT was still < 1 due to its excessive thermal conductivity (Figure 8). Despite this limitation, the notable power factor exhibited by Sr1−xLaxTiO3 proved its potential for application in thermoelectric systems [42]. Co-doping on both the Sr site and Ti site is a widely employed strategy in the field. In 2017, Wang et al. successfully overcame the challenges associated with nanoscale codoping of Nb and La by integrating the hydrothermal method with efficient sintering techniques. Using this approach, they achieved precise control over the co-doping Co-doping on both the Sr site and Ti site is a widely employed strategy in the field. In 2017, Wang et al. successfully overcame the challenges associated with nanoscale co-doping of Nb and La by integrating the hydrothermal method with efficient sintering techniques. Using this approach, they achieved precise control over the co-doping process, resulting in the precipitation of nano-inclusions during the sintering phase and the formation of a complex microstructure. Consequently, the Seebeck coefficient was significantly enhanced, while the thermal conductivity was effectively reduced. Finally, a record-high ZT of >0.6 was generated at 1000-1100 K in the 10 mol% La and 10 mol% Nb-doped SrTiO 3 bulk materials [46]. More recently, Acharya et al. proposed thermoelectric composites of Nb-doped SrTiO 3 using natural graphite, which possessed surged electrical conductivity, while restraining thermal conductivity (Figure 9). The ZT value can reach 1.42 at~1050 K and keep within >1.2 of a broad temperature range (850-1050 K) [68]. process, resulting in the precipitation of nano-inclusions during the sintering phase and the formation of a complex microstructure. Consequently, the Seebeck coefficient was significantly enhanced, while the thermal conductivity was effectively reduced. Finally, a record-high ZT of > 0.6 was generated at 1000-1100 K in the 10 mol% La and 10 mol% Nbdoped SrTiO3 bulk materials [46]. More recently, Acharya et al. proposed thermoelectric composites of Nb-doped SrTiO3 using natural graphite, which possessed surged electrical conductivity, while restraining thermal conductivity (Figure 9). The ZT value can reach 1.42 at ~1050 K and keep within > 1.2 of a broad temperature range (850-1050 K) [68]  In addition to common doping, low-dimensional is another effective strategy to improve thermoelectric performance. In thin film cases, phonon scatterings at the interface and surface areas will be much more obvious than in the bulk state. In 2021, Zhang et al. prepared La-and Nb-doped SrTiO3 full-range solid solution epitaxial films by the PLD method and investigated their thermoelectric phase diagrams at room temperature (Figure 10) [69,70]. Both SrTiO3-LaTiO3 and SrTiO3-SrNbO3 solid solutions displayed a similar power factor (=S 2 · σ) changing pattern, which peaked at the carrier concentration (n) of around 10 21 cm −3 , while different thermal conductivity variation tendencies, especially at the substitution level, were above 50 mol%. The authors attributed this to the different lattice distortions in the two solid solution systems. Modulation of the thermal conductivity through doping achieved a much larger ZT value of ~0.11 at room temperature compared to previous reports [71]. In addition to common doping, low-dimensional is another effective strategy to improve thermoelectric performance. In thin film cases, phonon scatterings at the interface and surface areas will be much more obvious than in the bulk state. In 2021, Zhang et al. prepared La-and Nb-doped SrTiO 3 full-range solid solution epitaxial films by the PLD method and investigated their thermoelectric phase diagrams at room temperature ( Figure 10) [69,70]. Both SrTiO 3 -LaTiO 3 and SrTiO 3 -SrNbO 3 solid solutions displayed a similar power factor (=S 2 · σ) changing pattern, which peaked at the carrier concentration (n) of around 10 21 cm −3 , while different thermal conductivity variation tendencies, especially at the substitution level, were above 50 mol%. The authors attributed this to the different lattice distortions in the two solid solution systems. Modulation of the thermal conductivity through doping achieved a much larger ZT value of~0.11 at room temperature compared to previous reports [71]. In 1993, Dresselhaus et al. published their findings that the two-dimensional electron system (2DES) can enhance thermopower without sacrificing electrical conductivity, which significantly boosted the ZT value [72]. As shown in Figure 11a, the absolute value of thermopower is proportional to the energy differential in the density of the state: In 1993, Dresselhaus et al. published their findings that the two-dimensional electron system (2DES) can enhance thermopower without sacrificing electrical conductivity, which significantly boosted the ZT value [72]. As shown in Figure 11a, the absolute value of thermopower is proportional to the energy differential in the density of the state: Molecules 2023, 28, 5894 10 of 1 Figure 11. Enhancing thermopower through the two-dimensional electron system (2DES In order to achieve a stronger enhancement effect, Dresselhaus et al. proposed the concept of using materials with longer de Broglie wavelengths in quantum well construc tions [74]. Based on this concept, Zhang et al. utilized the tunable de Broglie wavelength in the SrTiO3-SrNbO3 full-range solid solution when constructing superlattices [69]. With a longer de Broglie wavelength, the superlattice structures achieved superior overall ther moelectric performances over the bulk counterparts [75]. As shown in Figure 12a, the 2DES exerts a much higher enhancement factor (S2DES/SBulk) in the thermopower, where the 2DES with a longer de Broglie wavelength reached 10, which is ~2 times larger than the 2DESs, which has a shorter de Broglie wavelength. Meanwhile, a longer de Broglie wavelength better maintains the better electron-transport property. As a result, the highes power factor in the superlattices reached ~5 mW m −1 K −2 , thereby reaching 200% of the bulk counterparts (Figure 12b). As the thickness of the 2DES decreases below the de Broglie Wavelength, the conduction band (CB) will split, which results in an enhancement of thermopower. In 2007, Ohta et al. fabricated the SrTi 0.8 Nb 0.2 O 3 |SrTiO 3 superlattice structure and confirmed the 2DES enhancement effect in thermopower by decreasing the thickness of the quantum wells (Figure 11b) [73]. A ZT value of~2.4 was achieved in the 2DES (Figure 11c).
In order to achieve a stronger enhancement effect, Dresselhaus et al. proposed the concept of using materials with longer de Broglie wavelengths in quantum well constructions [74]. Based on this concept, Zhang et al. utilized the tunable de Broglie wavelength in the SrTiO 3 -SrNbO 3 full-range solid solution when constructing superlattices [69]. With a longer de Broglie wavelength, the superlattice structures achieved superior overall thermoelectric performances over the bulk counterparts [75]. As shown in Figure 12a, the 2DES exerts a much higher enhancement factor (S 2DES /S Bulk ) in the thermopower, where the 2DES with a longer de Broglie wavelength reached 10, which is~2 times larger than the 2DESs, which has a shorter de Broglie wavelength. Meanwhile, a longer de Broglie wavelength better maintains the better electron-transport property. As a result, the highest power factor in the superlattices reached~5 mW m −1 K −2 , thereby reaching 200% of the bulk counterparts (Figure 12b).
in the SrTiO3-SrNbO3 full-range solid solution when constructing superlattices [69]. With a longer de Broglie wavelength, the superlattice structures achieved superior overall thermoelectric performances over the bulk counterparts [75]. As shown in Figure 12a, the 2DES exerts a much higher enhancement factor (S2DES/SBulk) in the thermopower, where the 2DES with a longer de Broglie wavelength reached 10, which is ~2 times larger than the 2DESs, which has a shorter de Broglie wavelength. Meanwhile, a longer de Broglie wavelength better maintains the better electron-transport property. As a result, the highest power factor in the superlattices reached ~5 mW m −1 K −2 , thereby reaching 200% of the bulk counterparts (Figure 12b).  [77]. Ca, Sr, Ba, Pb, and La were introduced to induce a large lattice distortion and a huge mass fluctuation effect, which resulted in a minimum thermal conductivity of 1.17 W m −1 K −1 at 923 K. Using newer concepts combined with traditional oxide-based materials, a chance remains to further increase their thermoelectric performance. Layered cobalt oxides (A x CoO 2 ) exert a sandwich structure composed of CoO 2 and A-site cations by stacking layer by layer ( Figure 13). The conduction path is dependent on the CoO 2 layer, which displays a large Seebeck coefficient due to the low-spin state of Co 3+ [78]. It has been theoretically and experimentally proven that the spin entropy flow generated from the electronic conduction between Co 3+ and Co 4+ majorly contributes to the enhancement of the thermoelectric potential of A x CoO 2 -based oxide materials [78][79][80][81]. At high temperatures, the Seebeck coefficient (S) can be calculated based on the Koshibae equation:

Layered Cobalt Oxides (A
where k B is the Boltzmann constant, e is the electronic unit charge, g is the different possible ways in which electrons can be arranged in the orbitals of Co 3+ and Co 4+ ions, and n is the concentration of a given Co species. Additionally, g is the product of the spin degeneracy (g s ) and orbital (g o ) degeneracy: g = g s . . .g o , whereby g s = 2ζ + 1, where ζ is the ions' total spin number and g o is the number of valid permutations for distributing the electrons across its orbitals. Using electronic conduction through hopping in Na x CoO 2 , a spin entropy flow occurs in the opposite direction [82]. Therefore, the contribution of the spin entropy to the thermoelectric potential in the cobalt oxide group is related to the concentration of the Co 4+ and Co 3+ ions, as well as the degree of the spin degeneracy of Co ions. Since a decrease in the Co 4+ concentration will lead to an increase in spin entropy; thus, the manipulation of the Co 4+ ion concentration through excessive metal doping has emerged as an effective approach to modulating the spin entropy contribution. Using the ion exchange process, the A-site cations can be tuned from Na 2+ to Ca 2+ , Sr 2+ , and Ba 2+ and accompanied by a topotactic phase transition.
In 1997, Terasaki et al. discovered that Na0.5CoO2 crystal was a good cand thermoelectric application [35]. Compared to the traditional alloy-type materials was found that their power factors are in the same order of magnitude. Meanw conductivity of NaxCoO2 is nearly four times that of Bi2Te3 [35,83]. In terms of conductivity, the interface between layers affects heat transfer and contributes t duction in lattice thermal conductivity. Therefore, the large Seebeck coefficient a electrical conductivity also indicate that the layered Na0.5CoO2 crystal is a kind o based thermoelectric material with research significance and prospect. The disco also drawn the attention of researchers to various metallic layered cobalt oxide-ba moelectric materials.
However, constrained by the high thermal conductivity, the overall therm performance, ZT value, of Na0.5CoO2 is still far from the commercial level. In 2020 al. proposed that thermal conductivity can be suppressed significantly by substit site ions in AxCoO2 from light ions to heavy ions [84]. Using heavier A-site ions w a mismatch in phonon vibration modes between CoO2 and the A-site cation layer will reduce the phonon mean free path and phonon group velocity, and there thermal conductivity (Figure 14a). They fabricated AxCoO2 (Ax = Li1, Na0.75, Ca0 La0.3) epitaxial films using the PLD method followed by the reactive solid phase (R-SPE) method and ion exchange process. The thermal conductivity tendency w after following the hypothesis along the in-plane direction, suggesting that the t lectric performance of AxCoO2-based materials can be enhanced through an e modification of the A-site ions. In 1997, Terasaki et al. discovered that Na 0.5 CoO 2 crystal was a good candidate for thermoelectric application [35]. Compared to the traditional alloy-type materials Bi 2 Te 3 , it was found that their power factors are in the same order of magnitude. Meanwhile, the conductivity of Na x CoO 2 is nearly four times that of Bi 2 Te 3 [35,83]. In terms of thermal conductivity, the interface between layers affects heat transfer and contributes to the reduction in lattice thermal conductivity. Therefore, the large Seebeck coefficient and high electrical conductivity also indicate that the layered Na 0.5 CoO 2 crystal is a kind of oxide-based thermoelectric material with research significance and prospect. The discovery has also drawn the attention of researchers to various metallic layered cobalt oxide-based thermoelectric materials.
However, constrained by the high thermal conductivity, the overall thermoelectric performance, ZT value, of Na 0.5 CoO 2 is still far from the commercial level. In 2020, Cho et al. proposed that thermal conductivity can be suppressed significantly by substituting A-site ions in A x CoO 2 from light ions to heavy ions [84]. Using heavier A-site ions will create a mismatch in phonon vibration modes between CoO 2 and the A-site cation layers, which will reduce the phonon mean free path and phonon group velocity, and therefore, the thermal conductivity (Figure 14a). They fabricated A x CoO 2 (A x = Li 1 , Na 0.75 , Ca 0.33 , Sr 0.33 , La 0.3 ) epitaxial films using the PLD method followed by the reactive solid phase epitaxy (R-SPE) method and ion exchange process. The thermal conductivity tendency was good after following the hypothesis along the in-plane direction, suggesting that the thermoelectric performance of A x CoO 2 -based materials can be enhanced through an elaborate modification of the A-site ions.
Molecules 2023, 28, 5894 Figure 14. Reduction in thermal conductivity through heavy ion substitution at the AxCo (a) Schematic phonon propagation of AxCoO2. Heavy ions (sumo wrestler) display a stro mal conductivity suppression effect than light ions (baby). (b) Anisotropic thermal conduc AxCoO2. As the mass of A-site ions increases, the in-plane thermal conductivity (κ||) and cr thermal conductivity (κ⊥) decrease in tendency. Reproduced with permission [84].
Following this strategy, Takashima et al. checked the thermoelectric propert CoO2 (Ax = Na0.75, Ca0.33, Sr0.33, Ba0.33) epitaxial films along the in-plane direction shown in Figure 15, the power factor stays almost unchanged as the A-site ion creases, while the thermal conductivity is effectively suppressed, which suggests A-site ion exchange will not affect the electrical conduction path along the CoO2 l only reduce their thermal transport properties. It also needs to be emphasized tha shows an A-site ion arrangement evolution with temperature. For example, C exerts a phase transition from a √3 × √3 hexagonal superstructure to a 2 orthorhombic superstructure as the temperature increases above 400 °C [85]. How the Ba0.33CoO2 lattice, a mixture of orthorhombic and hexagonal superstructure maintained within a wide temperature range. This lattice disorder contribute cantly to phonon scattering and thermal conductivity reduction. A peak ZT value was realized in the Ba0.33CoO2 epitaxial film at room temperature, which is a rec among the reported "reliable" ZT values in conducting oxides. As the mass of A-site ions increases, the in-plane thermal conductivity (κ || ) and cross-plane thermal conductivity (κ ⊥ ) decrease in tendency. Reproduced with permission [84].
Following this strategy, Takashima et al. checked the thermoelectric properties of A x CoO 2 (A x = Na 0.75 , Ca 0.33 , Sr 0.33 , Ba 0.33 ) epitaxial films along the in-plane direction [40]. As shown in Figure 15, the power factor stays almost unchanged as the A-site ion mass increases, while the thermal conductivity is effectively suppressed, which suggests that the A-site ion exchange will not affect the electrical conduction path along the CoO 2 layer and only reduce their thermal transport properties. It also needs to be emphasized that A x CoO 2 shows an A-site ion arrangement evolution with temperature. For example, Ca 0.33 CoO 2 exerts a phase transition from a √ 3a × √ 3a hexagonal superstructure to a 2a × √ 3a orthorhombic superstructure as the temperature increases above 400 • C [85]. However, in the Ba 0.33 CoO 2 lattice, a mixture of orthorhombic and hexagonal superstructures can be maintained within a wide temperature range. This lattice disorder contributes significantly to phonon scattering and thermal conductivity reduction. A peak ZT value of~0.11 was realized in the Ba 0.33 CoO 2 epitaxial film at room temperature, which is a record high among the reported "reliable" ZT values in conducting oxides. As indicated in the report by Takashima et al., Ba0.33CoO2 is a promising thermoelectric material with superior performance to other counterparts. Later, Zhang et al. tested the high-temperature thermoelectric properties of AxCoO2 (Ax = Na0.75, Ca0.33, Sr0.33, Ba0.33) epitaxial films to confirm the application of Ba0.33CoO2 at elevated temperature environments [41]. They found a relatively high thermal stability of Ba0.33CoO2 up to 650 °C in air compared to other counterparts, by checking the room temperature resistivity after heat treatment in air at elevated temperatures (Figure 16a). High thermal stability makes it possible to measure thermoelectric properties of Ba0.33CoO2 up to 600 °C, with a ZT value of 0.55 at 600 °C (Figure 16b), comparable to those of PbTe and p-type SiGe (Figure 16c), which indicates Ba1/3CoO2 as a good candidate for high-temperature thermoelectric applications.

Summary and Perspective
The recent advancements in oxide-based thermoelectric materials, with a focus on ZnO, SrTiO3, and layered cobalt oxides, are reviewed, as they have garnered increasing attention due to their advantages over the chalcogenide family. The combination of high thermal/chemical stabilities and abundant elements on Earth makes thermoelectric oxides a highly promising candidate for numerous applications in waste heat recycling at elevated temperatures. Until now, these materials have witnessed revolutionary breakthroughs in performance enhancement through conventional doping and nanostructuring techniques. Additionally, the concept of low-dimensionalization has emerged as a new approach to further improve efficiency, especially in the context of device miniaturization As indicated in the report by Takashima et al., Ba 0.33 CoO 2 is a promising thermoelectric material with superior performance to other counterparts. Later, Zhang et al. tested the high-temperature thermoelectric properties of A x CoO 2 (A x = Na 0.75 , Ca 0.33 , Sr 0.33 , Ba 0.33 ) epitaxial films to confirm the application of Ba 0.33 CoO 2 at elevated temperature environments [41]. They found a relatively high thermal stability of Ba 0.33 CoO 2 up to 650 • C in air compared to other counterparts, by checking the room temperature resistivity after heat treatment in air at elevated temperatures (Figure 16a). High thermal stability makes it possible to measure thermoelectric properties of Ba 0.33 CoO 2 up to 600 • C, with a ZT value of 0.55 at 600 • C (Figure 16b), comparable to those of PbTe and p-type SiGe (Figure 16c), which indicates Ba 1/3 CoO 2 as a good candidate for high-temperature thermoelectric applications. As indicated in the report by Takashima et al., Ba0.33CoO2 is a promising thermoelectric material with superior performance to other counterparts. Later, Zhang et al. tested the high-temperature thermoelectric properties of AxCoO2 (Ax = Na0.75, Ca0.33, Sr0.33, Ba0.33) epitaxial films to confirm the application of Ba0.33CoO2 at elevated temperature environments [41]. They found a relatively high thermal stability of Ba0.33CoO2 up to 650 °C in air compared to other counterparts, by checking the room temperature resistivity after heat treatment in air at elevated temperatures (Figure 16a). High thermal stability makes it possible to measure thermoelectric properties of Ba0.33CoO2 up to 600 °C, with a ZT value of 0.55 at 600 °C (Figure 16b), comparable to those of PbTe and p-type SiGe (Figure 16c), which indicates Ba1/3CoO2 as a good candidate for high-temperature thermoelectric applications.

Summary and Perspective
The recent advancements in oxide-based thermoelectric materials, with a focus on ZnO, SrTiO3, and layered cobalt oxides, are reviewed, as they have garnered increasing attention due to their advantages over the chalcogenide family. The combination of high thermal/chemical stabilities and abundant elements on Earth makes thermoelectric oxides a highly promising candidate for numerous applications in waste heat recycling at elevated temperatures. Until now, these materials have witnessed revolutionary breakthroughs in performance enhancement through conventional doping and nanostructuring techniques. Additionally, the concept of low-dimensionalization has emerged as a new approach to further improve efficiency, especially in the context of device miniaturization and flexibility. With the implementation of novel strategies aimed at enhancing the performance of thermoelectric oxides, such as augmenting the power factor through low-

Summary and Perspective
The recent advancements in oxide-based thermoelectric materials, with a focus on ZnO, SrTiO 3 , and layered cobalt oxides, are reviewed, as they have garnered increasing attention due to their advantages over the chalcogenide family. The combination of high thermal/chemical stabilities and abundant elements on Earth makes thermoelectric oxides a highly promising candidate for numerous applications in waste heat recycling at elevated temperatures. Until now, these materials have witnessed revolutionary breakthroughs in performance enhancement through conventional doping and nanostructuring techniques. Additionally, the concept of low-dimensionalization has emerged as a new approach to further improve efficiency, especially in the context of device miniaturization and flexibility.
With the implementation of novel strategies aimed at enhancing the performance of thermoelectric oxides, such as augmenting the power factor through low-dimensionalization and reducing thermal conductivity via high-entropy alloying, oxide-based thermoelectric materials are poised to undergo significant advancements in the foreseeable future.